Born this way: What’s important for vertebrate reproduction?

How does reproduction differ across vertebrates?

Vertebrates (animals with a spine) perform reproduction and care for their young in different ways. This is part of life history theory, a framework of biological evolution that seeks to explain animal behaviour and anatomy (1). Scientists know of relationships between vertebrates and their life histories on a broad scale (1). For example, the larger a lizard gets, the more offspring it has, while the opposite is true for mammals, and size has not real effect on birds clutch size (Figure 1).

Figure 1. The number of offspring an animal has changes with body mass based on whether the animal is a reptile, mammal or bird. The litter size per mass of individual species are represented by circles. Image reproduced from Shai’s presentation on his recent publication (Meiri et al., 2020) at Macquariue University, 15 April 2020. (S1)

We know that several other factors can influence life history, but they haven’t been looked at on a global scale (1,2). Could something be going on at the evolutionary level across all birds, lizards and mammals around the world? Shai Meiri from Tel Aviv University, Israel created a database to compare life history of vertebrates on their reproduction strategies on a global scale (1,2). Shai’s work is the first to shed light on the evolutionary implications of life history on such a scale for vertebrates (1).

Reproduction strategies vertebrates differ in their reproductive strategy and life histories. Shai’s research looked at the following in particular:

1. Heat regulation. Animals can either be: (i) ectothermic (cold-blooded) reacting to environmental temperature, or (ii) endothermic (warm-blooded animals) spending energy to stay warm. Heat regulation frequently determines birth type and parental care (see Table 1).

2. Birth type. Many vertebrates are oviparous (egg-laying) but some have opted to become viviparous, where the embryo develops inside of the parent until live birth. Considering the breadth of the change from oviparity to viviparity, we might expect sizes and number of offspring to shift (2).

3. Parental care. Some animals provide food and warmth for their offspring (called parental care). fully independent (precocial), where they can move and find food immediately after birth. Whereas most endotherms are born helpless (altricial), and require parental care, primarily as they cannot produce enough warmth to keep alive (1).

4. Egg type. Vertebrates that appeared earlier (such as amphibians and fish) havesimpler eggs, making them anamniotes. More recent vertebrates, such as birds, reptiles and mammals developed a more complex egg (amniotes) which contains extra-embryonic membranes. These membranes help the egg survive stressful environments, and are a key adaptation that allowed animals to live fully terrestrial lives.

Video 1. The game-changing amnotic egg – April Tucker Learn more about the extra-embryonic membranes help animals develop into the first fully terrestrial vertebrates. By TED-Ed 19 June 2013. (Accessed 27 April 2020). (S2)

Table 1. Fish, reptiles, birds and mammals all differ in at least one way in these life history traits. Table created by Shannon Kaiser.

Viviparity, thermal and parental care are major transitions in reproductive strategies (1). The literature suggests that life history traits in viviparous, endothermic and show parental care will show the following changes (1,2).

Top 5 expected traits

1. Fewer babies are born at once
2. Babies are larger
3. Parents reproduce less frequently
4. Fewer babies are born per year
5. Lower maximum longevity (endotherms live shorter lives)

Surprisingly, none of these predictions came true.

Life history theory

Why didn’t we get the predicted results?

This study shakes the validity of fundamental aspects of life history theory (2). For example, a foundational theory in life history theory, is the rate-of-living theory, which predicts that as an animal’s metabolic rate (the rate at which an animal uses energy) increases, its life expectency decreases (2). This suggests that an endotherm, which uses energy to keep warm, would have shorter life expectancy than an endotherm (1,2). Yet, when we compare across all known bird, reptile and mammals, we find little evidence of this (2). Thus, while many studies (which often focus at a lower level, such as between bird species) find the theory to hold, when compared amongst a global scale the theory fails (1,2,3).

Looking at an even broader scale?

Life history theory is not limited to vertebrates. Do these theories be true for other broad animal groups? Studies have investigated life history in the biological group Arthropoda (4). Which contains insects, spiders, crustaceans, centipedes and millipedes. All of these groups show differing forms social behaviour, parental care, internal/external reproduction and so on (5,6,7,8). Would we see a similar lack of significance across these groups? Additionally… could a less studied biological trait influence life history? (9)

Miniaturisation.

Miniaturization, or evolution of extremely small body size, is widespread across all animal groups (9). Only recently (approx. 2005) gaining traction in the literature (9). It influences morphology, physiology, and behaviour of animals (9). Animals change in complexity in these areas as they become smaller – could some traits be more important to reproduction? (9,10) This is an interesting potential study that has yet to be performed, and the author hopes to see it in the literature in the future.

Future research

What else could be driving vertebrate reproduction? Shai looks at viviparity in squamates (scaled reptiles) in response to geography and temperature, (click here to read it when its published: Shai’s page).

References

1. MEIRI, S., FELDMAN, A., SCHWARZ, R. & SHINE, R. 2020. Viviparity does not affect the numbers and sizes of reptile offspring. Journal of Animal Ecology, 89, 360-369.
2. STARK, G., PINCHEIRA‐DONOSO, D. & MEIRI, S. 2020. No evidence for the ‘rate‐of‐living’theory across the tetrapod tree of life. Global Ecology and Biogeography.
3. GLAZIER, D.S., 2015. Is metabolic rate a universal ‘pacemaker’for biological processes?. Biological Reviews, 90(2), pp.377-407.
4. CHAPIN, K.J., 2017. Arthropod Life History. Encyclopedia of Animal Cognition and Behavior. Springer, Cham.
5. GILBERT, J.D. and MANICA, A., 2015. The evolution of parental care in insects: a test of current hypotheses. Evolution, 69(5), pp.1255-1270.
6. MITIC, B.M., STOJANOVIC, D.Z., ANTIC, D.Ž., ILIC, B.S., GEDGED, A. and MAKAROV, S.E., 2014, July. Parental care in centipedes (Myriapoda: Chilopoda): A phylogenetic perspective. In Tuf, IH & Tajovský, K.(Еds.), 16th International Congress of Myriapodology (pp. 20-25).
7. THIEL, M., 2003. Extended parental care in crustaceans–an update. Revista Chilena de Historia Natural, 76(2), pp.205-218.
8. YIP, E.C. and RAYOR, L.S., 2014. Maternal care and subsocial behaviour in spiders. Biological Reviews, 89(2), pp.427-449.
9. POLILOV, A. A. 2016. At the size limit-effects of miniaturization in insects, Springer.
10. HANKEN, J. and WAKE, D.B., 1993. Miniaturization of body size: organismal consequences and evolutionary significance. Annual Review of Ecology and Systematics, 24(1), pp.501-519.

Conference, image & video references

S1. MEIRI, S., 2020, ‘Scales feathers or fur? Babies or eggs? What matters for vertebrate reproduction?’ Biological Sciences Seminar Series, Macquarie University, Sydney, 15 April 2020

S2. TED-Ed (19 June 2013). The game-changing amnotic egg – April Tucker Available at: <https://www.youtube.com/watch?v=Qq0kMEWzdHg&gt; (Accessed 27 April 2020).